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          vue之计算属性&侦听器
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        <p>click look more</p>
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<h1 id="计算属性"><a href="#计算属性" class="headerlink" title="计算属性"></a>计算属性</h1><blockquote>
<p>设计模板表达式的初衷是为了简化计算，但是随着业务的深入，这里的代码就会变得复杂难以维护。代码如下：</p>
</blockquote>
<p>首先模板从vue实例中拿到message，然后进行一系列的操作。第一感觉就是很烦不简洁</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">div</span> <span class="attr">id</span>=<span class="string">"example"</span>&gt;</span></span><br><span class="line">  &#123;&#123; message.split('').reverse().join('') &#125;&#125;</span><br><span class="line"><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br></pre></td></tr></table></figure>

<p>那么如何处理呢？按照文档的话来说：<strong>处理任何复杂逻辑的运算都应该被放在计算属性中</strong></p>
<h2 id="基础例子"><a href="#基础例子" class="headerlink" title="基础例子"></a>基础例子</h2><blockquote>
<p>已经有过了解了，所以直接上代码。可以发现一个之前没有注意过的细节。就是Mustache中不只是可以放data属性里的东西，computed计算属性也可以。</p>
</blockquote>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">div</span> <span class="attr">id</span>=<span class="string">"example"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">p</span>&gt;</span>Original message: "&#123;&#123; message &#125;&#125;"<span class="tag">&lt;/<span class="name">p</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">p</span>&gt;</span>Computed reversed message: "&#123;&#123; reversedMessage &#125;&#125;"<span class="tag">&lt;/<span class="name">p</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> vm = <span class="keyword">new</span> Vue(&#123;</span><br><span class="line">  el: <span class="string">'#example'</span>,</span><br><span class="line">  data: &#123;</span><br><span class="line">    message: <span class="string">'Hello'</span></span><br><span class="line">  &#125;,</span><br><span class="line">  computed: &#123;</span><br><span class="line">    <span class="comment">// 计算属性的 getter</span></span><br><span class="line">    reversedMessage: <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">      <span class="comment">// `this` 指向 vm 实例</span></span><br><span class="line">      <span class="keyword">return</span> <span class="keyword">this</span>.message.split(<span class="string">''</span>).reverse().join(<span class="string">''</span>)</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>

<h2 id="computed-VS-method"><a href="#computed-VS-method" class="headerlink" title="computed VS method"></a>computed VS method</h2><blockquote>
<p>其实上面的例子也可以写在method中，那么有什么不同呢？</p>
</blockquote>
<p>computed：计算属性是依赖于data里面属性的变化而变化的。而他们又是响应式的，所以只要data不变，那么computed的值也就不变。所以在vue中，computed的值被<strong>缓存</strong>了下来。只要data不变，不论这个计算属性调用多少次，都会立即返回结果。即只在第一次被调用或者data改变时才会进行计算。</p>
<p>method：只要页面被渲染，method总会重新进行计算。</p>
<blockquote>
<p>所以可以明确，如果希望有缓存并且计算量大的话，就使用computed。反之则使用method</p>
</blockquote>
<h2 id="computed-VS-watch"><a href="#computed-VS-watch" class="headerlink" title="computed VS watch"></a>computed VS watch</h2><blockquote>
<p>这个不深究，之前写过一篇文章解释。文档核心意思就是，不要滥用watch</p>
</blockquote>
<h2 id="计算属性的setter"><a href="#计算属性的setter" class="headerlink" title="计算属性的setter"></a>计算属性的setter</h2><blockquote>
<p>计算属性默认只有 getter，不过在需要时你也可以提供一个 setter：</p>
</blockquote>
<p>现在再运行 vm.fullName = ‘John Doe’ 时，setter 会被调用，vm.firstName 和 vm.lastName 也会相应地被更新。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// ...</span></span><br><span class="line">computed: &#123;</span><br><span class="line">  fullName: &#123;</span><br><span class="line">    <span class="comment">// getter</span></span><br><span class="line">    <span class="keyword">get</span>: function () &#123;</span><br><span class="line">      <span class="keyword">return</span> <span class="keyword">this</span>.firstName + <span class="string">' '</span> + <span class="keyword">this</span>.lastName</span><br><span class="line">    &#125;,</span><br><span class="line">    <span class="comment">// setter</span></span><br><span class="line">    <span class="keyword">set</span>: function (newValue) &#123;</span><br><span class="line">      <span class="keyword">var</span> names = newValue.split(<span class="string">' '</span>)</span><br><span class="line">      <span class="keyword">this</span>.firstName = names[<span class="number">0</span>]</span><br><span class="line">      <span class="keyword">this</span>.lastName = names[names.length - <span class="number">1</span>]</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// ...</span></span><br></pre></td></tr></table></figure>

<h1 id="侦听器"><a href="#侦听器" class="headerlink" title="侦听器"></a>侦听器</h1><blockquote>
<p>虽然computed能满足大部分情况，但是<strong>需要在数据发生变化时进行异步操作或开销较大的运算时，就需要使用watch</strong></p>
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          <div class="post-toc motion-element"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#计算属性"><span class="nav-number">1.</span> <span class="nav-text">计算属性</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#基础例子"><span class="nav-number">1.1.</span> <span class="nav-text">基础例子</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#computed-VS-method"><span class="nav-number">1.2.</span> <span class="nav-text">computed VS method</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#computed-VS-watch"><span class="nav-number">1.3.</span> <span class="nav-text">computed VS watch</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#计算属性的setter"><span class="nav-number">1.4.</span> <span class="nav-text">计算属性的setter</span></a></li></ol></li><li class="nav-item nav-level-1"><a class="nav-link" href="#侦听器"><span class="nav-number">2.</span> <span class="nav-text">侦听器</span></a></li></ol></div>
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